How to prune a tree of selective nodes without recursion - swift

Question

I can't solve the following problem without recursion. I get that the solution has to do with making a list of nodes to process but that's where I get stuck.
The problem is to remove all nodes from a tree with a negative value. I wrote an example in Swift below and included the recursive solution I came up with. What is the solution to the problem without using recursion?
struct Tree: Codable {
enum CodingKeys: String, CodingKey {
case children
case value
}
let children: [Tree]
let value: Int
}
let tree = Tree(children: [
Tree(children: [
Tree(children: [], value: 3),
Tree(children: [], value: -1)
], value: 5),
Tree(children: [
Tree(children: [], value: 3)
], value: -2)
], value: 12)
func prune(tree: Tree) -> Tree? {
if tree.value < 0 {
return nil
}
return Tree(children: tree.children.compactMap { child in
prune(tree: child)
}, value: tree.value)
}
let prunedTree = prune(tree: tree)
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let json = try! encoder.encode(prunedTree)
print(String(data: json, encoding: .utf8)!)
// {
// "value" : 12,
// "children" : [
// {
// "value" : 5,
// "children" : [
// {
// "value" : 3,
// "children" : [
//
// ]
// }
// ]
// }
// ]
// }